1. Field of the Invention
The teachings generally relate to a non-destructive method of measuring a moisture content profile across a dimension of a hygroexpansive, composite material using radiation and a volumetric shrinkage correction.
2. Description of the Related Art
Moisture gradients in hygroscopic, composite materials, such as wood, are known to affect internal stresses that cause dimensional changes and eventual development of defects. Severe deformation and other defects of finished products reduce product quality and have the potential to damage a manufacturer's reputation and significantly increase the cost of manufacturing.
One of the oldest methods for measuring moisture gradient in wood was the bandsaw slicing technique. This technique is not entirely accurate due to the kerf and moisture losses caused by heat generated during high speed cutting. The methods of slicing thin layers were improved later by using a drill bit and a microtone knife. Since the samples were destroyed after measuring the moisture gradient, these methods failed to provide a continuous monitoring of moisture movements inside the samples. This made it difficult to develop accurate models of moisture gradient and movement during wood drying.
The search for a nondestructive method of measuring moisture in wood started in early 1900. The electrical resistance of wood was found to be a good indicator of its moisture content and was widely used after the extensive study on the electronic properties of wood. Instead of using single parameter (electrical resistance), many advanced devises employed radio frequency signals to measure moisture content in wood. These methods were based primarily on principles of electrical resistance, capacitance, and phase. Since the relationship between moisture content in wood and its electrical properties were not fully understood, the radiofrequency technique was dependent on the experimental data to create an empirical relationship for each species.
Warping of wood-based composites is a long-standing problem associated with secondary manufacturing processes, such as the drying of the wood products, in the wood panel industries. Severe warping of finished products has the potential to significantly increase the cost of manufacturing and lower the consumer's confidence in using wood composites. The Composite Panel Association (previously called the National Particleboard Association) considers warp to be the leading technical problem requiring further investigation.
Composite wood panels can be regarded as a multi-layered composite material, where each layer has a unique set of physical properties. Individual layers can be approximated as an orthotropic material having two principal directions. The mechanical behavior of layered composite materials is quite different from that of most common engineering materials which are homogeneous and isotropic. The makeup and physical properties of layered composites varies with location and orientation of the principal axes. Wood has unique and independent mechanical properties in the directions of three mutually perpendicular axes, so it may be described as an orthotropic material.
Linear expansion of wood composites usually includes thermal expansion and hygroscopic expansion. Both types of linear expansion perform about the same way, but hygroscopic expansion is more common and thermal expansion is relatively very small. Hygroscopic expansion is dependent on changes in moisture content, so moisture content change is the key parameter to determine the linear expansion once a panel is produced. Generally speaking, minimizing the moisture content change is the best way to keep panels from warping. For a panel with balanced construction, if the moisture content changes through its thickness are constant, there will be no induced out-of-plane stress. Therefore, there will be no warp. But for a panel with unbalanced construction, even if moisture content through the thickness changes uniformly, it is still possible to have out-of-plane deformation and warping problems. Balanced-construction composites are not necessarily free from warping. For a balanced panel, if there is an moisture content gradient through the thickness, the panel will still have a warping tendency.
Accordingly, one of skill will appreciate a non-destructive method of measuring moisture content, moisture content profiles, and the movement of moisture throughout a hygroexpansive, composite material such as wood. The wood product industry would benefit highly from a fast and accurate method of measuring moisture movement during drying processes, improving the design of drying processes, and preventing product damage associated with the removal of water during the drying of wood products.